The measure of James A. Baker III is that he came late to Ronald Reagan's camp as the commander of enemy political campaigns, tangled in the Reagan White House with some of the president's oldest and most influential supporters, incurred the wrath of Reagan's conservative core supporters, acted as the president's negotiator with his foes on Capitol Hill -- and wound up at the end of Reagan's first term as supreme honcho of the White House staff.

This sounds like the resume of a bold, gambling, swashbuckling political gunslinger, but the words Baker's admirers use in talking about him sound like the Boy Scout creed -- "cautious," "conservative," "orthodox," "consistent," "honest," "loyal," "decent," "talented" and "hard-working."

His critics in the New Right, who rejoice at his departure as White House chief of staff, would quarrel with only a few, beginning with "conservative," but they add a few of their own -- "opportunistic," "self-aggrandizing," "ruthless," a man with his eye primarily on the brass ring.

Baker's success at climbing to the apex of the American political system, according to his admirers, is due to his superb political instincts and intelligence. They note that Baker negotiated most of the legislative compromises that gave Reagan his political victories, particularly on the budget and tax cuts.

"He understands both macro- and micro-politics," one Republican campaign consultant said yesterday. "He understands broad themes and coalitions but he also know the nuts and bolts of running a campaign. He's a realist -- pragmatist is a dirty word in this party -- who practices the art of the possible.

"He has a great reputation in the Republican Party and good contacts. People trust him."

To his detractors, however, he is primarily a good in-house politician who negotiated the compromises on the Hill at the expense of conservative doctrine.

"He built his own stature at the expense of others in the White House," said one Republican campaign consultant. "Some think he's an excellent manager who runs a good shop, but apart from the mechanics he's not a strong Reaganite. He's a little too willing to compromise."

Baker came as a defeated enemy chieftain after the collapse of George Bush's presidential campaign in May 1980 to a political camp dominated by longtime friends and associates of Reagan's since his years as California governor in the late 1960s and early '70s. They had reason to be both wary of Baker and impressed by him.

He first broke onto the national political scene in 1976 as chief delegate hunter in President Gerald R. Ford's struggle with Reagan for the Republican nomination, which went all the way to the convention. He then was chairman of Ford's general election campaign, in which the incumbent closed from a 30-point deficit right after the convention to lose only narrowly to Jimmy Carter in both popular and electoral votes.

Baker, 54, a millionaire Houston lawyer, is the great-grandson of the founder of the Houston law firm Baker & Botts. He was graduated from Princeton, was commissioned a Marine Corps officer during the Korean war and then practiced in the family firm.

Baker was born and raised a Democrat but switched parties in 1969 to become Harris County (Houston) chairman for George Bush when Bush unsuccessfully ran for the Senate in 1970.

"I got religion from George Bush," Baker said.

Bush persuaded Baker to be Ford's delegate hunter and then to run Bush's 1980 presidential campaign, which got off to a spectacular start when he upset Reagan in the Iowa caucuses in late January. It came to grief five weeks later when Reagan won the New Hampshire primary and went on to win the nomination.

"Baker kept Bush alive because he stretched those resources further than anyone thought possible," a Republican campaign consultant said. "He spent wisely, followed a budget, which is practically impossible to do, and put enough money and emphasis into Iowa to win it. He's a superb administrator."

Reagan had no reason to be fond of Baker -- at the end of the 1976 campaign he was furious because he thought Baker was the source of accusations that Reagan campaigned halfheartedly for the ticket and therefore contributed to defeat -- but Baker made up for it at the end of the 1980 primary season.

Bush had swamped Reagan by about 2 to 1 in the Michigan primary in May, and a Yankelovich poll showed him ahead of Reagan and Carter.

"It was too late, but we had momentum," Pete Teeley, Bush's press secretary then and now, said yesterday. "Bush wanted to push on and take Reagan on in California."

But Baker had a longer view -- he didn't want Bush to jeopardize his chances of getting on the ticket. Right after the Michigan primary, without Bush's knowledge, he told reporters that Bush would have to reassess his campaign.

Baker was taken on the Reagan campaign shortly after and, after the election, became White House chief of staff.

"In the first year of the administration it was Baker who led the planning and put together the coalition on the Hill that passed the president's program," said David R. Gergen, former White House communications director and a Baker colleague. "He worked well with the president and enhanced the intellectual quality and integrity of the White House."

He also tangled with some longtime Reagan supporters. William P. Clark, now secretary of the interior, departed as White House national security affairs adviser partly because of clashes with Baker, but got revenge by scotching Baker's attempt to succeed him.

He had a run-in with William J. Casey, director of the Central Intelligence Agency, over the Reagan campaign's acquisition of Carter's briefing book for the debate with Reagan in the 1980 campaign. Baker said he got the book from Casey before the debate. Casey denied it.

Two years ago, Baker also called for the resignation of Labor Secretary Raymond J. Donovan, who has been plagued by allegations of organized-crime connections and now is under indictment on grand larceny and fraud charges.

"Ray Donovan shouldn't be here," Baker said in January 1983. " . . . He should do what's right for the president."